In the completion of a well, it is customary to use one or more casings or liners in a well bore. For each casing or liner, cement is pumped upwardly in the annulus between the outer wall of the casing and well bore to seal off the annulus and prevent the vertical migration of liquids along the casing in a well bore. For one reason or another, after the cementing job is completed it is not uncommon to have vertical channels or other voids in the cement in the annulus. Sometimes, the channels appear after production. In any event, in dual zones particularly, the production of water from a lower zone can migrate through a vertical channel and be produced from the upper zone. This is a highly undesirable situation.
Heretofore, cement squeeze techniques for remedial and intermediate cementing operations in well bores have been used in an attempt to seal off the channels in a cemented annulus. Typically, a squeeze cementing operation begins by isolating the perforated interval of a well casing traversing earth formations and thereafter pumping a cement slurry through a tubing and below a cement retainer into perforations which are located along an interval of casing. The remedial operation is intended to remedy a prior cement job which has been ineffective by filling any voids existing in the prior cement job. The voids in the cement annulus characteristically are vertical and located in random order about a casing. Heretofore, the perforations for a squeeze cementing job routinely consist of two to eight shots ranging from one-half to two feet. The azimuthal angle between perforations has typically been at 0.degree., 90.degree. or 120.degree. phasing and the entrance hole size of the perforation was not considered to be of great importance.
In the prior art techniques, the success ratio of sealing off vertical channels has not been too successful. It has been estimated that one square inch of cross-sectional area in a vertical channel can accommodate the flow of several hundred barrels of water per day. For example, with a one-half inch diameter perforation in a seven inch casing, the probability of intersecting a small vertical channel in a cement annulus with a 90.degree. phase gun is very small. Further, where a channel in a cement annulus contains mud or water and the perforation intersects the channel, the likelihood of introducing formation damage occurs during the squeeze operation.
Cement squeezing techniques involve a cement slurry which is pumped into a well casing through perforations in the casing. The cement slurry if successfully filling voids, may also come into contact with porous and permeable earth formations. At higher pressures, the water in the cement slurry is squeezed out of the cement slurry and partial hydraulic dehydration of the cement occurs. When the dehydrated cement becomes impermeable, it forms a seal in the annulus between the casing and the well bore. One of the problems associated with cement squeezing is the risk of damaging producing formations.
The likelihood of shutting off the water flow in a cement channel or void in the cement annulus also depends upon the squeeze cement first reaching the channel or void and thereafter flowing into the channel or blocking the channel. Heretofore, perforating the casing for the squeeze job in respect to the number of shots or their placement has never been a major consideration. Squeeze perforations in a casing typically consist of two to eight perforations placed in an interval ranging from one-half foot to two feet and an azimuthal angle between perforations of 0.degree., 90.degree. or 120.degree.. The probability of a typical perforating gun aligning with a channel or void in a cement annulus is not particularly great and the assumption that pressure will allow fluid to break into a channel is not always valid.
For example, if the channel has a one-square inch cross-section it may be capable of flowing several hundred barrels of water per day. Thus when the perforators produce perforations of one-half inch in size in four directions, only two inches of the pipe circumference are perforated. The circumference of a 7" casing is 21.99" and that of a 97/8" borehole is 31.02", which illustrates the small likelihood of intersecting a small vertical channel.
Assuming that a perforation penetrates a vertical channel in the cement annulus, there is a likelihood that the channel is filled with drilling mud or dirty water which possesses some of the characteristics of a cement slurry and will partially dehydrate at the interface of a permeable and porous foundation when subjected to higher pressure. Unless the channel continues to the surface, the applied pressure will cause dehydration and the formation of a low permeability barrier or seal at the channel and formation face so that the channel is hydraulically sealed to high volume flow rates. Under these conditions, neither the channel nor the perforation can transmit liquid at a high flow rate, but both transmit the resulting increase in hydraulic pressure to the formation. At some point the path of liquid flow is into the formation, where it breaks down or fractures creating a void where the mud, dirty water and/or cement slurries can be accommodated. The exact point where the liquids enter the formation is difficult, if not impossible, to determine, and the entry point may or may not be at the channel in the cement annulus.
The success or failure of a cement squeeze to shut off water does not preclude formation damage to a productive interval. The water squeeze from a cement slurry contains fine grain cement particles and will cause some formation damage by reducing the relative permeability if it enters the productive zone. The water alone may also reduce the permeability if the formation happens to be water sensitive. With repeated attempts to squeeze off water, the permeability can be continuously reduced and greater pressures be incurred so that the probability exists that the production could be shut off.